There is a paucity of human fetal ultrasound exposimetry data including the study of fundamental mechanisms leading to bioeffects. Our group has systematically collected and published in vivo human reproductive ultrasound exposimetry data heretofore not available in the literature. To date, we have focused our research efforts on the first half of human pregnancy and the current proposal extends our in vivo exposimetry investigations into the third trimester of pregnancy. The first three specific aims of our current proposal therefore are to use our established methods to: (1) directly determine from in vivo exposimetry measurements the in vivo insertion loss separately in each of the listed individual maternal tissue layers: (i) skin and subcutaneous fat, (ii) rectus fascia, muscle and peritoneum, and (iii) myometrium, using both imaging and Doppler ultrasound modalities; (2) directly determine from in vivo exposimetry measurements the total-path in vivo insertion loss when the ultrasound beam traverses through the maternal abdominal wall (skin, subcutaneous fat, rectus fascia, rectus muscle and peritoneum) and myometrium, using both imaging and Doppler ultrasound modalities; and (3) use the data from specific aims 1 and 2 to determine quantitatively how passage through the maternal abdominal wall layers and myometrium affects the characteristics of the incident ultrasound beam. In this process we will be able to: (l) estimate the two-dimensional in vivo field distribution of the diagnostic ultrasound image field; (2) estimate the two-dimensional in vivo temperature increase distribution of the diagnostic ultrasound image field. These studies will be performed in patients who are going to undergo a clinically planned, non-emergency, cesarean section after the infant has been delivered and uterine hemostasis has been ensured using our tested exposimetry system to obtain the individual layer and composite insertion loss data. We plan to accomplish the only previous specific aim we have not so far addressed: to determine the maximum values of ultrasonic quantities to which the human embryo and second trimester fetus are exposed during pulsed Doppler ultrasound examination in this requested award period, using our improved and upgraded instrumentation and without changing our published protocols. The long-term goal of these specific aims is the development of a generalized approach to the quantification of ultrasonic energy delivered to and absorbed by embryonic and fetal tissues in utero. These data will provide a fundamental basis for embryonic and fetal risk assessment associated with the use of diagnostic obstetrical ultrasound.